The user equipments (UEs) that are idle tune in to the base station at predetermined occasions, paging occasions, to check whether they are getting paged by the network. The reason for getting paged may for instance be that there is an incoming call for the UE to receive.
While in idle mode the UE is handling the mobility autonomously using neighbor cell information provided by the network. If the current serving cell becomes weak and there is a stronger neighbor cell, the UE will change serving cell to the stronger neighbor. During this so called cell reselection the UE is not monitoring paging and hence may miss if getting paged at that moment. To prevent that the paging is missed due to interruption caused by cell reselection, radio access networks are usually repeating the paging one or more times until the UE responds.
All base stations in a so called location (tracking) area for which the UE has registered are paging the UE. When the UE is reselecting to a cell in another location (tracking) area, e.g. due to crossing some geographical boundary, or changing to another radio access technology, it has to update the network on in which area it is, in what is called a Location (Tracking) Area Update procedure. During the time which the UE is updating the location (tracking) area, the radio access network will have outdated information on in which area to page the UE. To prevent that the paging is missed due to outdated location information, if the UE does not respond to paging in the registered location (tracking) area, the radio access network usually repeats the paging in adjacent location (tracking) areas.
The paging occasions follow a so called paging cycle which is configured by the radio access network node. The paging cycle length also depends on the radio access technology; see below for idle mode paging cycles for Global System for Mobile Communications (GSM), Wideband Code Division Multiple Access (WCDMA), Time Division-Synchronization Code Division Multiple Access (TD-SCDMA) and Universal Mobile Telecommunication Standard, Long Term Evolution (UMTS-LTE):
GSM: 471, 706, 942, 1177, 1412, 1648, 1883, 2118 ms
WCDMA: 640, 1280, 2560, 5120 ms
TD-SCDMA: 640, 1280, 2560, 5120 ms
LTE: 320, 640, 1280, 2560 ms
Circuit switched fallback (CSFB) is an interim solution for supporting voice calls to UEs that are connected to LTE until voice over LTE (VoLTE) and single radio voice call continuity (SRVCC) are supported in the networks. The UE can get paged in the LTE system on an incoming call in a legacy system, and can get redirected to the legacy Radio access technology (RAT) such as GSM, WCDMA, TD-SCDMA, etc. This means that a UE can safely camp on or be connected to an LTE cell without missing any incoming calls.
The UE gets informed about whether CSFB is supported in the LTE cell when carrying out a combined registration for circuit switched (CS) and packet switched (PS) services. If CSFB is not supported, the registration will fail. The standard-compliant UE action when CS is not supported is to deactivate the support for LTE.
CSFB requires upgrades of legacy networks, hence in areas where LTE networks are rolled out there might not always be CSFB support from the beginning. How fast and whether at all it will be supported depends on whether the operator is willing to invest in the legacy network. Various proprietary solutions have been introduced to allow UEs to camp on or be connected to LTE while at the same time being camped on a legacy RAT to monitor CS paging.
Simultaneous GSM/LTE (SG-LTE)
SG-LTE is a solution that allows simultaneous GSM and LTE activities by having two separate radios and one or two basebands. The UE can be engaged in LTE data traffic and at the same time support a voice call in GSM. A device supporting SG-LTE thus does rely on CSFB to allow camping on or being connected to LTE. SG-LTE can be considered a special case of dual Subscriber Identity Module (SIM) dual activity (DSDA) where both SIMs are from the same operator (physically a single SIM).
Single Radio-LTE (SR-LTE)
In SR-LTE a single radio is shared between LTE and a legacy RAT in a time-division manner. The UE is connected to or camping on LTE while at the same time camping on a legacy RAT. When for example monitoring paging in the legacy RAT, reading system information, carrying out mobility measurements, doing a location area update, or receiving a call, the radio is handed over to the legacy RAT and any LTE activities are getting punctured. A device supporting SR-LTE does not rely on CSFB to allow camping on or being connected to LTE. SR-LTE can be considered a special case of dual SIM dual standby (DSDS) where both SIMs are from the same operator (physically a single SIM).
Monitoring Legacy RAT Using Available Additional Receiver
A UE capable of carrier aggregation may use an available receiver otherwise reserved for a secondary component carrier to monitor paging, carry out mobility measurements and read system information in the legacy RAT. As long as there is large enough separation between LTE uplink (UL) and legacy RAT downlink (DL) spectrum, the legacy RAT can be received concurrently with LTE transmissions on the UL. Hence for this case the legacy RAT can be monitored without any impact on LTE performance.
However, in case that the spectral separation between LTE UL and legacy RAT DL is not sufficient, collisions between LTE UL transmissions and legacy RAT reception has to be avoided in order to prevent high energy leaking from the transmitter to the receiver and destroying the signal to be received. In many cases it will mean that LTE UL transmissions have to be skipped when in conflict with legacy RAT activities.
Depending on capabilities of the baseband and whether dual transmissions can be supported, it may also be possible to support functionality similar to SG-LTE with a single radio with two or more receivers.
For SR-LTE as well as for the approaches above, in case of too small spectrum separation, the LTE connection will be punctured at least partially during the time the legacy RAT is received.
Undesirably, when puncturing the LTE connection there will be a direct throughput loss due to that scheduled transmissions to and/or from the UE cannot be carried out since the radio is tuned to another frequency, and also, due to that hybrid automatic request (HARQ) acknowledges (ACKs) for received transport blocks immediately before the created gap cannot be transmitted, and hence the base station may retransmit the data although successfully received by the UE. The puncturing may also have an impact on the residual block error rate (BLER) leading to retransmissions in higher layers such as radio link control (RLC).
Depending on the link adaptation algorithm used by the base station, missed ACKs and/or channel quality reports due to the puncturing may lead to that the base station is lowering the modulation and coding scheme (MCS) when scheduling the UE—a so called backoff. As a result, the transmissions become more robust at the expense of a reduced throughput.